Current studies of intravenous injection of triiodothronine (T3) into thyroidectomized rats have provided evidence for a rapid extranuclear action by stimulation of mitochondrial oxidative phosphorylation. Hypothyroid status is evident by one month after thyroidectomy on a low iodine diet; serum T4 and T3 values are low or undetectable. Intravenous injections of minute doses (3 to 50 ng T3/100 g bw) were administered 30 minutes prior to sacrifice. Hepatic mitochondria were isolated rapidly: hormone injected rats were compared with veheicle injected controls with regard to ATP formation and oxygen cnsumption in the Clark oximeter. ATP formation was assessed by a 2 minute incubation of mitochondria in Tris-phosphate buffer containing (32P), stable ADP, and glucose and succinate as substrates. Hormone injection produced significant(P 0.01) enhancement of ATP formation ranging 130-200% with proportionate increase in Q02. P:O ratio was usually 2.0 signifying tightly coupled oxidative phosphorylation. Addition of dinitrophenol (10-5M) to the medium markedly reduced ATP formation. Stimulation was not abolised by injection of either cycloheximide (0.5 mg/100g bw) or actinomycin D (0.1 mg/100 g bw) one hour before sacrifice, signifying direct mitochondrial stimulation by T3 in the absence of new protein synthesis. Other current work has demonstrated displacement in vivo of tractor (125I)-T3 from mitochondrial receptor sites by loading doses of nonradioactive T3. The findings support direct hormonal stimulation of oxidative phosphorylation, mediated by specific mitochondrial T3 receptors. An integrated response to thyroid hormones is more likely than any solitary mode of action. Effects upon mitochondrial inner membrane and plasma membrane may represent initial actions. Binding by nuclear chromatin may be related to anabolic effects, not evident until after a lag period; such effects may be essential to normal growth, differentiation, and cell actions. Activation of mitochondrial energy metabolism seems a likely candidate for the earliest effects such as increased heart rate and oxygen consumption within a few hours after intravenous injection of T3 in human myxedema.